Many applications require compact coherent sources of radiation with extensive tuning ranges and high efficiencies. Injection seeding is a technology commonly employed to fulfill such requirements. By controlling the spectral properties of a power oscillator, referred to as slave, with an external low power output laser, referred to as seeder, the system efficiency and reliability can be improved while practical problems associated with high power lasers such as nonuniform pump profiles and thermally induced optical distortions or damages can be eliminated or reduced.
Single longitudinal mode (SLM) injection seeding has long been demonstrated as an effective approach to generating narrow linewidth of high power radiation and, in particular, to ensuring single transverse and longitudinal mode of either gain-switched or Q-switched operation. In conventional SLM injection seeding, a diode pumped solid-state (DPSS) monolithic ring laser or an external cavity diode laser or a fiber laser is employed as a seeder. In contrast with standing-wave cavities, ring lasers have the beam circulating in a loop, which eliminates problems such as spatial hole burning. The cavity length of the slave oscillator must be actively controlled to resonate at the injected frequency within the tolerance. Lasing will occur only in the desired longitudinal mode because the buildup time from the seed beam is much faster than any other unseeded modes that must build up from random noise photons.
SLM seeders can be operated at CW or pulsed mode. CW seeding is most commonly used because it eliminates the needs for timing between the seeder and the pump laser. SLM seed sources are normally based on short cavities to increase intermode spacing and require careful control of cavity length and/or using intra-cavity or extra-cavity etalons or gratings or other wavelength selective elements to filter out a desired single mode seed beam. Continuous tunability often relies on feedback control of the seeder cavity length, the crystal angles, and tuning mirrors covering a broad range of wavelengths. They are complicated and are limited to a small number of wavelengths. In addition, the seeds are generally too weak to produce high power single mode outputs.
One approach to producing high power single longitudinal mode outputs is based on multimode injection seeding. In U.S. Pat. No. 6,016,323, Kafka, et al. claimed an oscillator system, which produced a broadly tunable single longitudinal mode output from a multimode seed source and a short cavity resonator. Multimode seeders do not require cavity length control, however, the seeding may not be stable and the slave laser may suffer from mode hopping.
While some applications prefer laser emission on a single longitudinal mode, there exist other applications for which high optical quality beams, short temporal coherence length, high power output, and stable operation of multiple modes are desirable. Examples include laser optical scanning systems, optical memory devices, laser raster printing systems, laser display systems, inspection systems, lithographic systems, imaging instrumentation, and other applications where speckle reduction is necessary. In U.S. Pat. No. 5,974,060, Byren, et al. demonstrated a laser oscillator for simultaneously producing a number of widely separated longitudinal modes from a short cavity seeder. The optical length of the slave resonator cavity was adjusted to be an integer multiple of the optical length of the seed laser cavity. Although the resonant effect substantially reduces the threshold of seeding power, it requires a stringent and active control of the resonator cavity length to be resonant with the seed wavelength. Resonator length change may be a result of vibration or temperature variation, causing mode hop in a random manner. In a stable resonator, oscillation is generally limited to the fundamental, single transverse mode, TEM00, with a highly uniform intensity profile across it. However, the controlled size of the laser beam within the oscillator is very small, which limits the output power.
Alternatively, injection seeding can be operated under non-resonant conditions, e.g., detuning between the seed frequency and the slave cavity resonance, short slave cavity terminated by a weak reflector and/or non-resonant modulation, i.e. significant detuning of the round-trip period in the slave cavity from the seeder RF modulation period and its harmonics. For example, Rafailov, et al. demonstrated a tunable single mode operation from non-resonant self-injection seeding (IEEE Quantum Electronics 7, 2001) as well as a dual-wavelength (Applied Physics Letters 80, 2002) or multiple-wavelength (Applied Physics Letters 85, 2004) laser output from non-resonantly injection-seeded diode lasers.
In spite of these successes, the prior arts typically require complex and costly systems such as those employed for cavity length control and/or phase locking in order to synchronize pulse timing between the seed and seeded lasers. There is a need for compact, robust, reliable, efficient, and low-cost laser sources capable of generating wavelength-purified, stable and short-duration pulses with high power TEM00 output and low optical noise.